REPOSITORY > RESULTS

Abstract

Dielectric materials are essential components of electronic devices and electric power systems. Consequently, the design and development of new materials with enhanced dielectric response remain of great importance. This dissertation examines three different approaches to enhancing the dielectric response: (i) percolative composites, (ii) polymer blending, and (iii) the induction of criticality, each applied to a novel organic or ceramic system.
In light of ecological sustainability, polymer materials, particularly those based on cellulose, have gained significant attention. Due to their low dielectric permittivity, a composite approach is typically used to develop percolative polymers that exhibit a divergent response near the percolation threshold. In this work, titanium carbide MXenes were used as a filler instead of graphene oxide within a cellulose matrix. Measurements across broad frequency and temperature ranges revealed the influence of the preparation method and the type of nanofibrils matrix on the overall dielectric response, as well as the notable effect of absorbed water. A detailed investigation of vacuum-filtered cellulose nanofibrils/MXene composites, which contained the fewest impurities, confirmed that their dielectric response follows the predictions of percolation theory, resulting in a pronounced enhancement of dielectric permittivity with increasing filler content.
Another approach in polymer systems focuses on achieving higher electric energy density by operating at higher electric fields. Dielectric breakdown in polymers is usually initiated by space charges, which are accelerated by an external electric field in the free volume of the system. Therefore, blending appropriate polymers to increase chain packing density and reduce the number of space charges, through strong electrostatic interactions between oppositely charged polymer chains, is expected to enhance dielectric breakdown strength. Accordingly, poly(ether imide)/polyimide blends exhibited approximately 2.5 times higher dielectric breakdown strength than the pristine polymers, contained fewer space charges, and absorbed less water due to higher chain packing density.
Despite containing environmentally hazardous lead, lead-based ceramics remain dominant due to their superior functional properties. A physical approach to induce the maximum dielectric response at a critical end point has produced exceptional functional properties in ferroelectric and relaxor systems. In this work, the existence of two critical end points is shown for the first time in antiferroelectric ceramics using the model system Pb0.99Nb0.02[(Zr0.57Sn0.43)0.92Ti0.08]0.98O3. Moreover, enhanced energy storage density and dielectric tunability were observed near these critical end points. To determine the origin of the enhanced dielectric tunability (approximately 375 %) at the temperature of the field-induced triple point, minimization of domain size was proposed as the responsible mechanism and confirmed by Rayleigh measurements and piezoresponse force microscopy.
The results obtained in these novel systems provide new insights into the correlation between structural evolution and property changes in materials developed via different approaches, which is important both fundamentally and practically for developing new materials for advanced dielectric applications.